Waste water treatment using aerobes is advantageous in that the apparatus for retaining microorganisms within a reactor is less restricted in design as compared with treatment using anaerobes; for aerobes are more proliferative than anaerobes. Waste water treatment using aerobes is roughly divided into a suspension system using microorganisms suspended in waste water and an organic membrane system using microorganisms immobilized on a solid carrier. While various methods and apparatus for these systems have been proposed to date, the suspension system involves a problem that the floc of microorganisms has a low sedimentation rate and sometimes overflows the apparatus, failing to secure a requisite amount of the microorganisms (called a bulking phenomenon), which leads to reduction in treated water quality. To the contrary, the organic membrane system is free from such a problem and also achieves high efficiency in waste water treatment. From this standpoint, the latter system tends to be adopted in preference to the former system.
In spite of the above advantages, the organic membrane system essentially requires an intermittent cleaning operation to prevent clogging of the reactor with the proliferating microorganisms. To this effect, the treatment must be suspended to clean the reactor by air jetted from a cleaning pipe provided beneath the bottom of the reactor. However, some kinds of carriers or some structures for holding the carrier make it difficult to conduct cleaning properly while securing a sufficiently effective surface area of the carrier.
In more detail, in a system in which spherical or irregular-shaped inorganic molded or natural fillers as a carrier are merely packed in a reactor, the fillers contact to each other only to provide a small effective surface area, and with small spaces among themselves, they are liable to be clogged. In a system in which a cylindrical carrier holder having a large surface area, such as a honeycomb structure, is placed in a reactor, the effective surface area decreases as the microorganisms proliferate on the inner surface of the structure, and the structure is not easy to clean. In a system in which a fibrous or ribbon-shaped metallic or plastic material is supported by a column or a cage, interstices among fibers or ribbons, where densely entangled, are soon filled with the microorganisms, resulting in considerable reduction in effective surface area. Additionally, if this system is cleaned vigorously, the microorganisms fall off the carrier excessively to markedly reduce the treating capacity after the cleaning operation, failing to continue the treatment in a stable manner.
The intermittent cleaning is also accompanied by a serious disadvantage that the cleaning pipe is easily clogged during the treatment. Further, the air jetted from the cleaning pipe usually goes up almost vertically. In a common reactor whose height is about 4 to 5 m, the air flow diffuses to a diameter of about 30 cm at the most. Accordingly, in order to apply the air jet flow to every part of the carrier, it is necessary to densely provide a large number of nozzles for spouting air. This not only incurs increased initial cost and operating cost but makes operation control difficult.